Nerves got the better of a Martinborough freediver who came up four metres short of a world record today.

Martinborough architect Kathryn Nevatt, who can do without breathing for more than seven minutes, today attempted to break the women's freediving world record in the dynamic apnea without fins, set at 160 metres in Denmark in 2009 by Russian Natalia Molchanova.

Unofficially she already has, with a swim of 163 metres.

In a Masterton pool at 10.40am today she set out on her first attempt, surfacing at 156m - 4m short of the record.

''The nerves got the better of me,'' she said, adding she felt more confident about a second attempt tomorrow.

''I knew when to come up [today]. If I had done another stroke I probably wouldn't have come up.''

To officially hit the mark she needs to stay underwater for six lengths of the pool without taking a breath, while being watched by Los Angeles-based International Association for Development of Apnea judge Grant Graves.

Masterton District Council is the major sponsor of the event and has flown Mr Graves from the United States to adjudicate with other judges from around New Zealand.

Councillor Gary Caffell said if the public event went well, Masterton could become a regular fixture on the national and international freediving calendar.

New Zealanders excel in competitive breath-holding disciplines. Hawke's Bay-raised William Trubridge holds the world record in the free immersion and constant weight without fins disciplines, and was the first person to break the 100m depth barrier unassisted.

Nevatt's ritual before the big swims involves meditating in her wetsuit in the pool as she slips into a cocoon of positive thoughts, inner calm and a sleep-like trance where tension is washed away.

"I visualise my dive. Then I just essentially try to go to sleep by slowing my heart rate and reducing anxiety and being very relaxed."

Expectations are that even if she beats the record early she will continue trying to set new marks during the weekend and into next week.ATTEMPT TIMES